An air cleaner for the contemplated use conventionally comprises a housing having air inlet and outlet ports, an outer, principal filter cartridge and an inner, safety filter cartridge. Typically, the outlet port is central or axial, and air flows inward radially through the cartridges. The latter are usually mounted in the housing by means of a central support yoke, being clamped between an end of the housing and a closure member centrally traversed by an extremity of the yoke.
This is a sturdy and practical arrangement, and one which is quite satisfactory when the filter cartridges are substantial enough to tolerate the necessary axial compressive forces without deterioration. For this purpose, the filter cartridges, in which the medium itself may be pleated paper, are so constructed that the paper is contained between inner and outer cylindrical supporting and protective shells of perforated metal, extending between end caps like the paper medium itself.
It becomes increasingly desirable, however, to make at least the principal filter cartridge without the perforated metal shells. These cartridges are simply removed and discarded when they become filled with dirt, and the cost of this procedure is considerably increased if metal shells must be discarded with the paper medium. Since the safety filters seldom require replacement, the expense of providing them with inner and outer shells is not intolerable.
In the co-pending application of Michael Fox, Ser. No. 171,658, filed on July 24, 1980 and assigned to the assignee of the present application, an arrangement is shown in which an outer filter cartridge is provided with flexible end lips and means are provided for gripping the flexible lips by contact with the ends of the inner filter, so that the outer filter is not compressed axially but simply gripped at the lips for support. This arrangement is somewhat inconvenient to assemble, and the present invention comprises an improvement thereover.